Optical discs having the recording density of about 50 GB can be currently produced on a commercial basis even for general use in accordance with standards for Blu-ray Disc (BD) and High-Definition Digital Versatile Disc (HD DVD) using blue-violet semiconductor laser. In future, the optical discs are also desired to have as large a capacity as 100 GB to 1 TB of the Hard Disc Drive (MD).
However, in order to realize such super-high density optical discs, the high-density technique using a new system different from the conventional technique like BD using short wavelength and high numerical aperture (NA) of objective lens is required.
Attention has been paid to holographic recording technique using holography to record digital information while the storage technique for the next generation is being studied. The holographic recording technique is the technique of superimposing signal beam having page data information modulated two-dimensionally by a spatial light modulator and reference beam in a recording medium to modulate the refractive index in the recording medium by an interference pattern produced at the time of the superimposition, so that information is recorded in the recording medium (storage medium).
In reproduction of information, when the recording medium is irradiated with the reference beam used in recording, the holograms recorded in the recording medium act like a diffraction grating to produce diffracted beam. The diffracted beam is reproduced as the same light as the recorded signal beam containing phase information.
The reproduced signal beam is detected two-dimensionally at high speed by means of an optical detector such as CMOS and CCD. Such holographic recording technique can record two-dimensional information (page data) in the optical recording medium by a single hologram all at once and reproduce the information. A plurality of page data can be written in a certain place of the recording medium in a multiplexing manner and accordingly a large capacity of information can be recorded/reproduced at high speed.
A holographic recording technique is described in JP-A-2004-272268 (patent document 1), for example. This publication describes a so-called angularly multiplexed holograms recording system in which signal beam is focused on an optical information recording medium by a lens while the recording medium is irradiated with reference beam of paralleled light beam to interfere with each other so that holograms are recorded and further different page data are displayed in a spatial light modulator while an incident angle of the reference beam on the optical recording medium is different, so that angularly multiplexed recording is performed.
This publication describes the technique in which signal beam is focused by a lens and an aperture (spatial filter) is disposed in a beam waist thereof, so that the space between adjacent holograms can be shortened so that the recording density and capacity can be increased as compared with the conventional angle-multiplexed recording system.
Further, another holographic recording technique is disclosed in WO 2004-102542 (patent document 2), for example. This publication describes an example using a shift multiplexing system in which light from pixels on inner side is used as signal beam and light from pixels in ring-shaped zone on outer side is used as reference beam in a single spatial light modulator and both light beams are focused on an optical recording medium by the same lens so that the signal beam and the reference beam interfere with each other in the vicinity of the focal plane of the lens to record holograms.
Ian Redmond: Technical Digest ODS (2006), MA1. (non-patent document 1) discloses that when information recorded in an optical information recording medium is reproduced, phase conjugate beam of reference beam is used, so that optical components of signal detecting camera for recording system can be used as reproduction system.
Moreover, JP-A-2002-170247 (patent document 3) describes a holographic recording/reproducing method in which a holographic recording medium has a reflecting layer and phase conjugate beam obtained by reflecting reference beam by the reflecting layer is used to reproduce information recorded in the holographic recording medium.
According to the recording medium utilized in this recording/reproducing method, since the holographic recording medium (holographic storage medium) has the reflection layer, a galvano-mirror for obtaining the phase conjugate beam and a driving part for driving the galvano-mirror can be omitted to avoid height and size of the holographic recording/reproducing device from being increased.